The long-term goal of the proposed research is to delineate intrarenal mechanisms for the regulation of sodium transport. The working hypothesis is that sodium transport through the paracellular pathway of the proximal tubule is influenced by blood pressure and that the permeability of the tight junction of the proximal tubule is regulated by prostaglandins. The relationship between proximal sodium transport, renal interstitial hydrostatic pressure (RIHP), and tight junction permeability will be studied by in vivo microperfusion. The efflux and influx of the extracellular fluid marker, lanthanum, between the renal proximal tubule lumen and interstitium will be used as a measure of paracellular transport and tight junction permeability. The specific questions to be addressed are: 1) What are the relative contributions of net transepithelial water flux (Jv and of diffusional permeability of lanthanum through the tight junction to the paracellular flux of lanthanum; a) To determine the dependence of paracellular transport of lanthranum on Jv, net fluid reabsorption by the proximal tubule will be inhibited by adding an impermeant solute, mannitol, to the luminal fluid. b) To determine the role of electrodiffusive forces, the paracellular flux of the anion, ferrocyanide, will be measured and compared to the flux of the cation, lanthanum. 2) Does increased RIHP increase paracellular backflux? Lanthanum influx from the interstitium and efflux from the proximal tubule lumen before and after direct renal interstitial volume expansion (DRIVE) will be determined. 3) What hormonal mechanisms mediate changes in tight junction permeability of the proximal tubule? Paracellular transport, tight junction permeability, and proximal reabsorption will be determine in the presence and absence of a) inhibition of the cyclooxygenase pathway, b) inhibition of the cyclooxygenase pathway during DRIVE, and c) inhibition of the cyclooxygenase pathway during DRIVE with prostaglandin E2 or I2 replacement. 4) What cellular mechanisms mediate changes in tight junction permeability of the proximal tubule? Paracellular transport, tight junction permeability, and proximal reabsorption will be determined a) in the presence and absence of the cAMP agonist, (Sp) cAMPS, or the cAMP antagonist, (RpcAMPS, b) in the presence and absence of (Sp)cAMPS during inhibition of the cyclooxygenase pathway, and c) in the presence and absence of (Rp)cAMPS after DRIVE. These studies should provide an understanding of basic mechanisms underlying the regulation of sodium reabsorption by the proximal tubule and specifically the mechanism underlying the pressure natriuresis phenomenon which in turn is fundamental to the regulation of arterial blood pressure.